The easiest way to install extensions is via the built-in "Extension Warehouse" (which you can be found in "Window"). There you can find the extension "SketchUp STL", which is needed to im- and export *.stl files. Unfortunately, it is only possible to import single files, not several at once. However, this is a general SketchUp problem and is not limited to this file extension.

Attention: To install extensions over the Extension Warehouse, you need a free Trimble account!

However, not everything is available through the manager. A large platform for further extensions is called sketchUcation. Also here you need a free account.